The novel compound [Cu(meso)]3[Co(CN)6]29.5H2O was prepared and characterized structurally, magnetically and spectroscopically (UV-vis, FTIR and EPR). The crystal structure is result of the self-assembly of three [Cu(meso)]2+ and two [Co(CN)6]3- units leading to a polymeric zigzag chain. Isolated [Cu(meso)(H2O)2]2+ units counterbalance the charge of the whole structure. The crystal packing shows a large hollow space in the structure and a significant number of water molecules are found in these cavities. Magnetic properties exhibited a very weak antiferromagnetic exchange interaction between the copper ions, with J = -0.3 cm-1. FTIR spectrum showed typical frequencies of n(O-H) at 3604 cm-1 and bridging and non-bridging n(CN) at 2184 and 2128 cm-1, respectively. The electronic spectrum of the title complex presented a broad and symmetric band at 547 nm, assigned to the overlaid z2→ x2-y2, xy → x2-y2 and (xz, yz) → x2-y2 transitions. EPR data at 77 K presented a typical axial spectrum with g// = 2.33 and g⊥ = 2.01, in accordance with the tetragonal distortion of the copper(II) ion as supported by the single-crystal X ray diffraction data.

There has been a growing progress in the design of supramolecular architectures based on cyanometallate complexes for their relationship to molecular magnetism and electronic properties.1-15 These building blocks, combined with coordinatively unsaturared systems such as cationic Schiff-bases, produce 1D, 2D or 3D network structures. These assemblies, also called Prussian blue analogues, gained importance due to the innumerous possibilities to prepare novel bimetallic extended lattices.

Most of these structures contain two different transition metal ions. Cyanide bridges are linear and suitable to communicate spin density as some compounds behave as high-temperature molecular magnets.5 The study of electronic and magnetic properties is necessary to a better understanding of potential metal-metal exchange interactions. These properties are intimately related to the structure and are a result of the nature of the building blocks, their relative orientation in the crystal lattice and interactions such as H-bonding and van der Waals. Therefore, controlling preparative methods is a key step in obtaining complexes with the desired structural and physical properties.16,17

Our continuing interest in the tetradentate macrocyclic ligand meso = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (Scheme 1) encouraged us to investigate the reaction of [Co(CN)6]3- with [Cu(meso)]2+.2,18,19 Herein we report the synthesis and characterization of the new trans-cyano-bridged [Cu(meso)]3[Co(CN)6]29.5H2O coordination polymer.

Experimental

Physical measurements and instrumentation

X-ray diffraction data collection was performed on an Oxford-Diffraction GEMINI-Ultra diffractometer (LabCri) using graphite-Enhance Source MoKa radiation (λ = 0.71069 Å) at T = 120(2) K. Data integration and scaling of the reflections were performed with the Crysalis suite.20 Final unit cell parameters were based on the fitting of all reflection positions. Empirical multi-scan absorption corrections using equivalent reflections were performed with the program SCALE3 ABSPACK.21

The structure was solved by direct methods using the SHELXS program.22 The positions of all atoms could be unambiguously assigned on consecutive difference Fourier maps. Refinements were performed using SHELXL based on F2 through full-matrix least square routine.23 Most of the hydrogen atoms, including those bound to nitrogen atoms and those in water molecules, could be identified in the final Fourier difference maps. In the final model refinement those bound to carbon were geometrically positioned and refined using a riding model;24 for the others, distance restraints were imposed.

The magnetization measurement was performed on a Cryogenic SX-600 SQUID magnetometer in the temperature range 2-298 K with an applied field of 1000 Oe. Diamagnetic correction was evaluated from Pascal's constants.

UV-Vis spectra in the range 190-820 nm were obtained on a SHIMADZU UVPC 2401 spectrophotometer in dmf solutions.

X-band electron paramagnetic resonance (EPR) spectra were recorded on a Bruker ESP300-E instrument from dmf frozen solutions at 77 K.

Infrared spectra were obtained with a FTS3500GX Bio-Rad Excalibur series spectrophotometer in the region 4000-400 cm-1 in KBr pellets.

Materials

Reagent grade chemicals were used in this work. The macrocyclic meso was prepared as published elsewhere.19

The crystal structure of [Cu(meso)]3[Co(CN)6]29.5H2O is the result of the self-assembly of three [Cu(meso)]2+ and two [Co(CN)6]3- units. Figure 1 shows the molecular assembly, where each [Co(CN)6]3 linked up to the two nearest [Cu(meso)]2+via cyano bridges with the following atomic bond distances: Co-C2 1.891(4) Å, C2-N2 1.155(5) Å, N2-Cu2 2.612(3) Å, Co-C5 1.887(4) Å, C5-N5 1.158(8) Å, N5-Cu3 2.737(3) Å. The average non-bridging CN distance is 1.152(3) Å. The CoIII ion lies in a fairly octahedral environment. Mean values for bond lengths of Co-Caxial and Co-Cequatorial are 1.886 and 1.903 Å, respectively, while C2-Co-C5, N5-C5-Co, C5-Co-C1 angles are 178.20(14)º, 176.6(3) and 87.23(14)º in this order. The CuII ion suffers a strong tetragonal distortion as is evidenced by the much longer Cu-Naxial bond distances (Cu2-N2 2.612(3) Å and Cu3-N5 2.737(3) Å) when compared to the Cu-Nequatoral values, in average 2.040(3) Å. The Cu2 and Cu3 ions lie very far apart, with a Cu...Cu distance of 9.609 Å, whereas for adjacent Co...Cu distance is 5.245(5) Å. The asymmetric unit for each string is given by [(meso)Cu-NC-Co(CN)5]- and the charge between two of them is balanced by one isolated [Cu(meso)(H2O)2]2+ unit. Bond distances and angles in this work are close to the values reported by Mondal and co-workers for [Cu(dmpn)2]3[Co(CN)6]212H2O, which also shows a zigzag conformation of alternate [Co(CN)]6]3- and [Cu(dmpn)2]2+ ions (dmpn = 1-dimethylamino-2-propylamine).14

The cell unit reveals (Figure 2) the formation of polymeric zigzag chains with a very bent C5-N5-Cu3 angle of 132.08(3)º. In the [Cu(meso)]2+ units, each Cu atom is in an inversion center in the P-1 space group and, consequently, only half of the N and C atoms are non equivalent; the Cu and the four N atoms are on perfect planes. The crystal packing also shows a large hollow space in the structure, which accommodates a significant number of water molecules. There is an extended H-bonding network between the hydrogen and nitrogen atoms of terminal cyanide ligands. Crystallographic parameters and details of data collection and refinement are given in Table 1 and selected bonds lengths and angles are given in Table 2.

Other linear chain arrangements have been previously described in the literature. For example, Triki and coworkers26 reported on the [{Cu(tnH)2(H2O)2}{Fe(CN)6}]2H2O (tn = 1,2-diaminopropane). Two copper(II)-N4O2 units are connected via hexacianoferrate(II) bridges, where cyanides are in a trans configuration and the iron ions show an almost perfect octahedral coordination. An interesting case is seen for [{(323)2Cu2(Cl)Co(CN)6}n]2nH2O (323 = N,N'-bis(3-aminopropyl)ethylenediamine), which forms 1-D zigzag chains along the crystallographic axes a and b. The propagation is completed by hydrogen bonding interactions between adjacent strings of atoms to form a highly symmetric 3-D network.27

Magnetic properties

Magnetic measurements were carried out in the temperature range 2-298 K. The plot of χM as a function of temperature is displayed in Figure 3. The effective magnetic moment, µeff, at room temperature was determined as being 2.45 µB, close to the expected spin-only value (2.83 µB) for two non-interacting copper(II) (S=1) ions and the diamagnetism of [Co(CN)6]3 (low spin cobalt(III) ion, S = 0). The trinuclear [Cu(dmpn)2]2[Co(CN)6]ClO43H2O complex reported by Mondal and co-workers exhibited a µeff of 2.50 µB.14 From room temperature down to 3.0 K, µeff remains practically constant, corresponding to two non-interacting CuII ions. Below this point, µeff drops abruptly and reaches 2.20 µB at 1.5 K and under an applied field of 1000 Oe. Analysis of the molar susceptibility was done using the isotropic Heisenberg-Dirac-van Vleck model with the Hamiltonian H = -2JS1S2 and the theoretical expression for χ is given elsewhere.28

χM = [C(1-p)(2e2x)/1+3e2x] + ¼Cp + T.I.P. with x = |J|/kT.

Temperature-independent paramagnetism (TIP) was considered to be 1.00 × 10-6 mL mol-1. The expression of χM was corrected to include the fraction p of any paramagnetic impurity present and the Weiss term θ to account for any intermolecular interactions. Non-linear fittings carried out by varying J, g, p and θ led to estimated values for parameters of the theoretical expression to the experimental data.

Best fitting lines shown in Figure 3 yielded J = -0.30 cm-1 and g = 2.13 in accordance with a very weak antiferromagnetic interaction between the copper ions. The magnetic interactions are practically insignificant, mainly due to the large value of the Cu...Cu separation (9.609 Å). Also, the magnetic orbital of the copper(II) ion is of the type dx2-y2, with x and y axes defined by the short Cu-N bonds, and the axial positions, which are occupied by cyanide ions, have a low-spin density. Although no further experimental support is available, intermolecular interactions between copper(II) in the chain and the isolated [Cu(meso)]2+ can not be ruled out as the Cu...Cu separation in this case is slightly lower, 8.467 Å.

The infrared spectrum in KBr is consistent with the structural data presented and has molecular features of the macrocyclic ligand and the cyanometalate core. The occurrence of lattice water in the crystal structure is consistent with the band at 3604 cm-1. Frequencies of the CN stretching mode provide information about the ligand-bridging mode. The high intensity and lowest-frequency ν(CN) band at 2128 cm-1 was assigned to the equatorial terminal cyano ligands and the low intensity high-frequency vibration at 2184 cm-1 to the intermetallic bridging cyanides. For comparison see the polynuclear complex [Cu(dmpn)2]3[Co(CN)6]212H2O, that shows very similar values with bands at 2130 and 2180 cm-1.14 Further, [Fe(tmphen)2]3[Co(CN)6]2 (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) exhibits bands at 2127 and 2167 cm-1, respectively, while the free [Co(CN)6]3-ν(CN) band appears at 2126 cm-1.5

The electronic spectra of [Cu(meso)]2+ in Figure 4A shows a low intensity structureless broad band centered at 587 nm, typical of copper(II), and has contributions from three ligand-field transitions: z2→ x2-y2, xy → x2-y2 and (xz, yz) → x2-y2.31 The [Cu(meso)]3[Co(CN)6]29.5H2O compound exhibited a more symmetric band at 547 nm with a higher intensity, consistent with a significant change in the environment around the copper(II) ion after reacting with [Co(CN)6]3-. Complex [Cu(dmpn)2]3[Co(CN)6]212H2O shows a band with a maximum at 590 nm with the same assignment.14

EPR spectrum in dmf at 77 K (Figure 4B) exhibits a four line EPR spectrum for Cu2+, I = 3/2, with parameters g// = 2.33 and g⊥ = 2.01. Since g// > g⊥, the unpaired electron resides in the x2-y2 orbital. These results are consistently interpreted as being due to a tetragonal distortion around the copper(II) ion, which was also evidenced in the crystal structure by the longer Cu-Naxial bond lengths, as discussed above. The hexacyanocobaltate ion is diamagnetic and EPR silent.

Conclusions

In conclusion, hexacyanocobaltate(III) reacts with [CuII(meso)](ClO4)2 in water producing a 1D-polymeric aggregate formed by two units of [CuII(meso)]2+ bridged by one (µ-CN)CoIII(CN)53 anion. Single crystal X-ray diffraction analysis revealed a zigzag arrangement of Co-CN-Cu atoms and a packing space in the crystallographic framework filled with water molecules. The copper(II) centers (S=1/2) are too far apart and the solid features a small antiferromagnetism. EPR, UV-vis data and bond distances are well in accordance with a significant tetragonal distortion at the copper center.